1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for producing the same. More particularly, it relates to a magnetic recording medium which comprises a magnetic layer with improved surface smoothness, which has good electromagnetic conversion properties and is suitable for high density recording.
2. Description of the Prior Arts
Generally, magnetic properties of a magnetic recording medium are improved, for example, by orienting acicular magnetic powder having a magnetic axis of easy magnetization in the longer axis of the acicular particles along a longitudinal direction of the magnetic layer. However, in a magnetic recording medium which utilizes magnetic components in the longitudinal direction of the magnetic layer, an increase of the recording density has its limit. That is, as the recording density increases, the strength of a demagnetizing field in the magnetic layer increases so that remnant magnetization decreases and rotates, whereby detection of the recorded signals becomes difficult.
In a perpendicular magnetic recording system which utilizes a magnetic component perpendicular to the plane of the magnetic layer, the strength of the demagnetizing field decreases as the recording density increases, so that such a recording system is recognized to be suitable for high density recording. It is proposed to utilize hexagonal system ferrite magnetic powder or acicular magnetic powder and to orient its magnetic axis of easy magnetization in a direction perpendicular to the magnetic layer by applying a homogeneous magnetic field perpendicular to the magnetic layer by an N-S opposed type magnet and to vertically record signals by utilizing the perpendicular magnetic component of the magnetic medium (cf. Japanese Patent Kokai Publication No. 86103/1980). The magnetic recording medium produced by such method has a squareness ratio of up to 0.9 in the direction perpendicular to the magnetic layer, when the demagnetizing field which has influences on the properties of the magnetic recording medium in the perpendicular direction to the magnetic layer is compensated on a hysteresis curve measured by a sample vibrating type fluxmeter. However, when the squareness ratio exceeds 0.7, the magnetic recording medium produced by the above method does not show such a tendency that reproducing output of the magnetic recording medium and D.sub.50 at which reproducing output decreases to a half increase in proportion to the increase of the squareness ratio as found in the range of 0.7 or less of the squareness ratio. According to this method, when the squareness ratio exceeds 0.7, the increase of the recording output and D.sub.50 cannot be correctly measured. Therefore, the squareness ratio of 0.9 found by this measuring method does not necessarily mean that the squareness ratio of 0.9 in the vertical direction is sufficiently high. Then, the magnetic particles could not be sufficiently oriented in the direction perpendicular to the magnetic layer, and the magnetic recording medium could not be suitable for high density recording.